The invention relates to a process for obtaining C.sub.2+ or C.sub.3+ hydrocarbons from gaseous mixtures that contain hydrocarbons, by pressure scrubbing with a physical solvent selective for C.sub.2+ and, respectively, C.sub.3+ hydrocarbons, the resultant loaded solvent being then regenerated and reused.
It is known from DAS No. 1,114,475 to use a physical scrubbing step for removing a readily soluble component from a gaseous mixture with the aid of a selective solvent in such a way that this component is scrubbed out in the upper portion of the scrubbing column and, in the same column, in the lower section, an inert gas is employed for stripping from the solvent, other components having a lower solubility than the desired gas component dissolved in the solvent. The inert gas employed as the stripping gas in this process shows no solubility, or is only sparingly soluble, in the solvent utilized. A special feature of the scrubbing step in the conventional process resides in that a portion of desired component is added to the stripping gas in order to avoid partial stripping off of the desired component from the solvent.
Stripping of more sparingly soluble components from a loaded scrubbing fluid with an inert gas is possible if either the gas component which is to be dissolved in the solvent has such a high solubility that any amount of inert gas simultaneously dissolved in the solvent is negligible, because the dissolved gas does not need to meet high purity requirements with regard to the inert gas employed, or the inert gas may permissively be a constituent of the remaining, scrubbed gas.
According to one embodiment of DAS No. 1,114,475, a butadiene-butene gas mixture is scrubbed with a solvent wherein butadiene is readily soluble in the chosen solvent. Nitrogen is used as the inert gas and the solvent employed is dimethylformamide (DMF). In accordance with the solubility data in Landoldt Boernstein, volume "Technik" [Technology] 4th part, issue C, 1976, pages 268, 276, 277, the following solubilities apply for these gases in DMF at 20.degree. C. and under a partial pressure of 0.1 bar:
Butadiene: =39 Nm.sup.3 /t bar PA1 Butene: =19 Nm.sup.3 /t bar PA1 Nitrogen: =0.058 Nm.sup.3 /t bar
The resultant solubility ratio of butadiene:nitrogen=39:0.058=672. On the basis of this ratio, the influence of the dissolved nitrogen on the purity of butadiene, obtained as the product, can be ignored.
The relationships are different in the production of heavy hydrocarbons, for example C.sub.2+ hydrocarbons, from natural gas or other gaseous mixtures subjected to scrubbing with an organic solvent, as described, for example, in U.S. Pat. No. 4,526,594. If, for example, C.sub.2+ is to be obtained from natural gas in such a system the methane content in the product C.sub.2+ must lie below 1 vol-%. Normally, only methane is contained in natural gas as the lighter component.
Stripping off the methane dissolved in the solvent after the scrubbing step with the aid of an inert gas, such as nitrogen, is not possible since extremely strict calorific value conditions must be met by the head product from the scrubbing step. Consequently, dilution of methane with nitrogen is not permissible. Therefore, in the case of obtaining C.sub.2+ by means of a physical scrubbing operation, the principle of stripping off other components from a solvent containing dissolved product gas, as described in DAS No. 1,114,475, cannot be applied.
It is also known to strip off poorly soluble components with product gas in a combined scrubbing-stripping column, the stripping gas being generated by heating the loaded solvent. Such a method is disclosed, for example, in DOS No. 3,247,773. The loaded solvent is heated under pressure and the thus-released gas is recycled into the scrubbing column. During this step, at least part of the lesser soluble CO.sub.2, which is dissolved along with H.sub.2 S, is stripped off.
The mode of operation of stripping off more sparingly soluble components from a loaded scrubbing liquid is, however, only possible under pressures lying below the critical pressure of the scrubber bottom liquid. In the case of a CO.sub.2 /H.sub.2 S mixture, the critical pressure is very high, normally above 73 bar, so that in this case no drastic limitations arise. However, in the case of a hydrocarbon mixture, such as those which usually occur in natural gases, the critical pressure is in the range of from about 30 to 45 bar. Accordingly, the conventional stripping methods operating under pressure can be utilized only with great limitations, inasmuch as the customary pressures for natural gases fed into a pipeline system are in a range of 70 bar.
Based on this restriction of the pressure range, processes, such as the one set forth in U.S. Pat. No. 4,526,594, for obtaining heavy hydrocarbons from natural gas by a physical scrubbing step recommend expanding the loaded solvent and then compressing the thus-liberated gases to such an extent that they can be fractionated in a demethanizer or deethanizer to the required product purity. However, separation by distillation, due to the expenditure in apparatus, is extremely cost-intensive and also disadvantageous from an energy standpoint since the liberated gas is obtained without being pressurized and therefore must be compressed.